This makes absolutely no sense. I have 2 kernels: one working only with USB boot (GRUB) and the second one that works with both USB boot (GRUB, only plain non-UKI boot tested) and NVMe boot (systemd-boot, only UKI works). The configs should be the same. 
USB boot only: https://github.com/anonymix007/linux-x1e/commits/wip/x1e80100-6.11-rc5/ or package at https://github.com/anonymix007/x1e-alarm/releases/tag/packages
Both: https://github.com/anonymix007/linux-x1e/commits/wip/x1e80100-6.11-rc5-backup or package at https://github.com/anonymix007/t14s-alarm/releases/tag/packages
Here are the UKIs (uki.tar.xz): https://drive.google.com/drive/folders/1dICKG4FJ5LXDTbZypCvuTy3LYi6-ea5a
Can could be the issue here?